Conventionally, an electric heating device has a pot body 50 and a cover 51, see FIG. 5. The pot body 50 includes an outer casing 54 made of composite resin, an aluminum inner casing 52 and a heat source 53 which heats the pot body 50 from a bottom of the body. The object in the pot body is heated electrically. Some heat is lost from the through hole 55 at the bottom, the cover 51 and the outer casing 54. Air is filled between the outer casing 54 and the inner casing 52. The heat will transfer out to air by heat convection. Furthermore if the temperature difference between the outer side of the pot body 50 and the inner side of the pot body 50 is great, the heat dissipation is also greater. Although the prior art structure can retain heat therein, the effect is low. For a long time, the heat will lost. Thus the prior art heating device is not effective and is not power-saved.